1. Field of the invention
The present invention relates to a bar code reader for reading bar codes.
2. Description of the prior art
The basic structure of a bar code reader according to a first example of the prior art is shown in FIG. 4. Shown in FIG. 4 are the bar code label 1, plural light sources 2, and a cylindrical lens 3 for focusing the light emitted from the light sources 2. Also shown are the projector P which includes the light sources 2 and cylindrical lens 3, and a receptor Q, which includes a detector reflecting mirror 4, imaging lens 5, and image sensor 6. Circuitry 7 is also provided inside the case 8. Note that the light path from the light sources 2 to the image sensor 6 is shown by the dot-dash line in FIG. 4.
FIG. 5A and FIG. 5B show the distance (path length) from the image sensor 6 of the receptor Q to the bar code label 1. Assuming that the bar code label I is a mirror, the image sensor 6 may capture anything that falls in the area between lines L1 and L2. Such an area is called a mirror reflection area. In other words, the mirror reflection area is an area viewed from the image sensor 6 and reflected on the bar code label 1. Depending on the material of the bar code label 1, the surface of the bar code label 1 may be smooth as with mirrors, or irregular, as with paper. A smooth bar code label surface, may result in regular reflection or total internal reflection, causing the image sensor 6 to capture an image of articles that falls within the mirror reflection area.
Note that the distance (path length) from the image sensor 6 to the bar code label 1 shown in FIG. 5A is three times the path length shown in FIG. 5B. Note, further, that the detector reflecting mirror 4 has been deleted from FIG. 5A and 5B for simplicity only, and the relationship between the bar code label 1, imaging lens S, and image sensor 6 in FIGS. 5A and 5B is the same as that shown in FIG. 4.
If the light sources 2 are placed within the mirror reflection area, i.e., the area between lines L1 and L2, in FIG. SA or 5B, light from the light sources 2 is reflected from the bar code label 1 with total or nearly total internal reflection, provided that the surface of the bar code label 1 is smooth and resistant to irregular reflection, such as when the label is printed on a laminated paper. Because light reflected with total or nearly total internal reflection is stronger than irregularly reflected light, the output of the image sensor 6 is saturated as shown by peaks J and K in FIG. 6A. When this occurs the information contained in the bar code label 1 cannot be correctly determined, and bar code reading is disabled.
Note that the waveform shown in FIG. 6A was obtained using a linear image sensor as the image sensor 6 and two LEDs as the light sources 2 disposed in the mirror reflection area. The positions of the light sources 2, as seen from the front of the bar code reader generating the image sensor output voltage shown in FIG. 6A, are shown in FIG. 6B where 2J and 2K are the LEDs.
To avoid the light source mirror reflection area of bar code label 1, the light sources 2 are located outside the mirror reflection area.
Referring to FIG. 5B, the front (beginning) of the bar code label 1 as seen from the bar code reader during bar code reading is point A. The bar code end is point B. The distances from points A and B to point b at distance D1 from point A are lengths L7 and L8. The distances from points A and B to point c at distance D2 from point A are lengths L3 and L4. In FIG. 5A the distance from point A to point a is L5, and the distance from point B to point a is L6.
When the luminance distribution obtained by the bar code reader of FIG. 5A is considered as one standard luminance distribution, the same standard luminance distribution is obtained in the case of FIG. 5B when the light source 2 is located at point c. Specifically, in the case of FIG. 5A, the distance from the image sensor 6 to the bar code label 1 is 3 M, and the light sources 2 are located at point a. In the case of FIG. 5B, the distance from the image sensor 6-to-the bar code label 1 is M, and the light sources 2 are located at point c. Point c is selected at such a point as to satisfy the same distance ratio as L6/L5. Thus, L6/L5=L4/L3. If the light sources 2 are placed at any point which does not have the same distance ratio as L6/L5, such as at point b in which L6/L5&lt;L8/L7, the same standard luminance distribution will not be obtained.
To avoid mirror -reflections, conventional bar code readers have been constructed as shown in FIG. 4 with an increased distance (path length) from the image sensor 6 to the bar code label 1 (first construction), or as shown in FIG. 7 with a shortened distance (path length) from the image sensor 6 to the bar code label 1 (second construction). In this second construction the light sources 2 are located outside the mirror reflection area farther from the center of the reading aperture.
The problems with the conventional bar code readers shown in FIG. 4 and FIG. 7 include a large physical shape, a poor aesthetic design, and greater bar code reader weight resulting in poor operability.
Another general problem associated with increasing the distance (path length) from the image sensor 6 to the bar code label 1 with the design shown in FIG. 4 is that good surface precision and installation precision are required with the detector reflecting mirror 4 because of limitations on the case design and component placement. These requirements result in increased bar code reader cost.